The thermal processing of semiconductor wafers generally involves a number of heating steps in which the temperature is a critical parameter. One of the major problems in the thermal processing of wafers is to maintain temperature uniformity over the complete wafer since temperature non-uniformity affects not only the proper thermal processing of the wafer, but also the proper measurement of the wafer temperature since the temperature is usually measured at the wafer center. This problem is particularly acute in Rapid Thermal Processes (RTP) frequently used to reduce the wafer heating time. In RTP, the wafer is heated by lamps, typically tungsten-halogen lamps, at a very high heating rate, e.g. approaching or exceeding 100.degree. C./sec. Such a high heating rate is to be sharply contrasted with the 1-2.degree. C./min in regular furnaces which heats the wafer by gas, or 10-50.degree. C./min in mini-furnaces. Non-uniformity is an inherent problem of RTP because of: (1) the temperature differences between the wafer and other parts of the reactor which are not heated; and (2) the fast heating which does not provide sufficient time to equalize temperatures.
A major cause for temperature non-uniformity is due to the heat losses at the outer edge of the wafer. Thus, the wafer is generally supported on a head in the heating chamber via a rim and a high emissivity ring for blocking stray radiation from the heater which might influence measurements of the wafer temperature. The rim radiates a substantial amount of heat from its side, and also conducts heat via the high emissivity ring to the head. As a result, the wafer edge becomes cooler than the center.
One method to solve the problem is to vary the power of the heater as a function of the wafer radius, i.e., to concentrate more heat from the heater on to the outer edge of the wafer. However, this method of reducing temperature non-uniformity is very limited because of the difficulty in focusing more heat on the outer edges of the wafer than at the center.